I will do one of two things.
If it's a matter of interpersonal stress (like conflict in the workplace or an argument with someone), my MO is to turn into a doormat. I just bend right on over and let people walk all over me for the sake of avoiding conflict. But if they continue to push (especially if it's about something that matters to me or if they're challenging my INFP "values"), then I go into shadow mode and I become an outright cruel, malignant dictator. I use that special NF ability to pinpoint a person's weakness and I go for the jugular. I can be straight up vindictive, verbally assaulting someone from every angle: personal, professional, physical, you name it. Honestly, I think it tends to shock people more than anything else, but it always gets them to listen to me and do what I have to say. Not that I really feel any better having been so horrible...but that's what happens when you keep pushing my buttons. It's not something I'm proud of, or even realize I'm capable of most of the time, but that's the "unhealthy" demon when it comes out.
If it's more of a personal stress situation (like personal responsibilities I can't handle or feelings I'm having that are overwhelming me) I tend to withdraw. I become really anti-social, don't want to leave my house, don't want to call my friends. I neglect cleaning, don't answer the phone, stop getting my work done, etc. I basically cease to be a functional human being and prefer instead to lie in my bed listening to sad music and wallowing in self-pity while thinking, with the utmost conviction, that no one else could possibly understand the way I feel or feel as deeply as I do. Wow, writing that out makes it seem so pathetic, but it's the sad truth. The worst part about this behavior is that it only gets worse the more I indulge in it. That's when I need a friend to intervene and bring me out of my funk. I haven't figured out how to stop myself in these situations, but I have at least learned to get a close friend on the phone so they can get me out of that bed and back into real life before it's too late!
Anyway, I'd guess that those are two common behavior patterns to look out for in an INFP. If they're under a lot of stress, they're likely to be hypersensitive (and in turn hypercritical or even cold and domineering) for brief and intense moments. They're also very likely to be overwhelmed by their feelings, systematically shut down, and withdraw. These are just some guesses based on my own personal experiences, but I could be wrong. Any other INFP's wanna offer insight?